Floating semi-submersible oil production and storage arrangement

ABSTRACT

An arrangement for the storage of marketable quantities of crude oil at a semi-submersible floating production vessel. The storage is achieved by hanging a segmented reinforced concrete tank to the underside of the semi-submersible vessel. The semi-submersible vessel can be an existing semi-submersible drilling rig. By maintaining the mass of the tank and contents slightly greater than the displacement of the tank and by arranging the centre of gravity of the tank below its centre of buoyancy, the metacentric height of the semi-submersible vessel is approved. The storage arrangement for the oil provides the necessary maintenance of mass by either storing approximately ⅘ of the oil in oil-over-water chambers and approximately ⅕ in gas-over-oil chambers or by using a gas-over-oil-over-water arrangement in all the chambers. The piping arrangements minimize the free surface of liquids in the tank.

The oil industry frequently makes use of floating production and storagesystems for developing small remote oil fields. These generally use aconverted crude oil tanker moored to a purpose built mooring buoy. Tominimize the wave, current and wind forces on the tanker, the mooring isdesigned to allow the vessel to weathervane around the buoy under theinfluence of the resultant environmental force. Processing equipment ismounted on the deck of the vessel.

Oil production from the reservoir is via one or more sub sea wellheads,through a flexible flowline from each wellhead to the mooring buoy andfrom the buoy to the vessel. The system also incorporates lines to carrygas and/or water from the vessel to the wellheads. Also incorporated arehydraulic and electrical lines from the vessel to allow control of thesub sea wellheads. Since the vessel must be free to rotate around thebuoy, the numerous fluid flow paths through the buoy result in the needfor a complicated and expensive device known as a swivel. This is aprecision-engineered piece of equipment subject to high pressure, hightemperature corrosive fluids from the reservoir and hence must bemanufactured faultlessly if high maintenance costs are to be avoided.

A further disadvantage of floating production systems that employ oiltankers is that they are highly susceptible to pitching, rolling andheaving. Since the separation of the oil, water and gas which comprisethe fluid stream from the reservoir is effected by means of gravityseparation in large pressure vessels, the sloshing of the liquids causedby the ship's motion can cause serious inefficiencies in the separationprocess.

An alternative type of floating production system which eliminates theseproblems is use of a semi-submersible vessel. Semi-submersible vesselshave been used in the offshore industry for a number of years as mobiledrilling vessels, crane barges, pipelaying vessels and dedicatedfloating production vessels. As shown in FIG. 1, a semi-submersiblevessel comprises a deck 1 supported above the waterline (OWL) on anumber of columns 30. The columns extend from the deck to (typically)two flotation pontoons 31 located some distance below the water line.The advantages of a semi-submersible over a ship-shaped vessel aretwo-fold. Firstly, the area exposed to the waves at the water line isless for a semi-submersible than for a ship-shaped vessel and hence thehorizontal wave forces are reduced. Secondly, because the pontoons whichprovide the buoyancy are much further below the water line than theunderside of a ship, the vertical forces are much less. (This is becausethe effects of a wave rapidly decrease as one moves deeper into thewater.)

The results of these advantages are that semi-submersible productionvessels can be moored in the ocean without the need to provide weathervaning and that the sloshing of liquids in vessels on the deck isreduced.

Semi-submersible floating production systems (SSFP systems) however havetwo disadvantages. Firstly, there is no significant capability forstorage of the produced oil. This means that they can only be utilizedwhere a pipeline is provided to carry the produced oil to an onshorestorage/processing facility or where a dedicated moored tanker ship isprovided adjacent to the SSFP vessel.

The second disadvantage is that the amount of processing equipment whichcan be fitted on deck is limited because the centre of gravity of theSSFP vessel is raised as weight is added to the deck. This reduces theresistance to overturning of the vessel. This resistance to overturningis quantified in a property of the vessel known as the metacentricheight (usually designated GM). A high GM means a high resistance tooverturning.

A number of oil fields have been developed using a SSFP vessel whichhave used a converted second hand semi-submersible drilling vessel.Where the produced oil is viscous and needs large pressure vessels forseparation or where gas injection or water injection equipment isrequired, new larger semi-submersibles are required to accommodate theequipment.

A number of attempts have been made to provide oil storage in asemi-submersible (British Patent Applications GB22116849, GB2207892,GB2188291 for example). However, these allow storage of only arelatively small quantity of oil. These systems still require adedicated moored tanker to store a marketable quantity of oil. Thestorage provided in the semi-submersible vessel only provides storage ofa few days production to allow the storage tanker to travel to a nearbyrefinery for offloading.

According to one aspect of the present invention there is provided anoil storage assembly for a semi-submersible oil production vesselcomprising a deck structure, at least two underwater pontoons forproviding buoyancy to said deck structure, and a plurality of columnsconnecting said deck structure to said pontoons, characterised in that aconcrete tank is attached below said pontoons, said concrete tank beingsubdivided into a plurality of chambers for storing fluid.

An arrangement for the storage of oil in accordance with the inventionhas the advantage that it provides a system for storing large quantitiesof oil which is not as susceptible to extreme environmental conditions,does not decrease, and in fact may be configured to increase theresistance to overturning of a rig with which it is used, and is easilymaintained in situ.

More particularly, the present invention preferably provides asemi-submersible, floating production, storage and offloading system forthe development of offshore oil and gas fields comprising a drillingvessel, an oil storage assembly according to the invention attached tothe base of the drilling vessel, means for utilizing the drillingvessel's ballast pumps to add or remove water from the bottom of eachchamber of the tank and means for directing produced oil into or out ofthe top of each chamber.

The present invention further provides a method of storing oil in anoffshore floating oil production facility comprising the steps ofattaching to the bottom of a pontoon structure a concrete tank, which issubdivided into a plurality of chambers, filling said chambers with atleast one fluid to adjust the buoyancy of the production facility, anddisplacing said fluid from said chambers by pumping produced oilthereinto in a controlled fashion such that the mass of the tank and itscontents is maintained substantially constant.

Preferably, the concrete tank is divided into a number of chambers, inparticularly, by a plurality of fluid tight bulk heads, at least one ofwhich chamber is located substantially centrally of the tank and is openat the top and bottom so as to provide a through opening in the tank. Inthis way, a rig can carry out drilling or workover operations with thetank attached.

Preferably, each chamber of the tank is maintained full of at least onefluid at all times so as to control the ballast of the arrangement. Thefluids used in this way may, for example, be sea water, oil, natural gasor a mixture of two or more of these.

At least some of the chambers preferably include a first inlet/outletpipe which terminated substantially at the bottom of the chamber and asecond inlet/outlet pipe which terminated substantially at the top ofthe chamber. In particular, at least some of the chambers preferablyinclude a water inlet/outlet pipe and an oil inlet/outlet pipe. Eachsuch chamber is then at least partially filled with water prior tocommencement of oil production, that water being displaced from thechamber through the water pipe as oil is added to the chamber by meansof the oil pipe. In the same way, oil may be evacuated from the tank forexample to a tank for transport to shore, by displacement using waterinjected through the water pipe. The water pipe advantageouslyterminates close to the bottom of the chamber, since sea water isnormally denser than oil, and preferably has a diffuser pipe on the endthereof so as to minimize the mixing of oil and water as water is pumpedinto the chamber. The oil pipe then preferably terminates at the innerface of the upper surface of the chamber, which arrangement has theadvantage that it avoids the possibility of a gas pocket building up inthe tank.

It has been found to be particularly advantageous for maintaining themass of the tank and contents substantially constant as oil is producedor offloaded if a first plurality of chambers operate this oil overwater storage arrangement, and a second plurality of the chambersoperate with either oil, natural gas or a mixture thereof storedtherein. In particular, each of the second plurality of chambers ispreferably provided with a first pipe which terminates proximate to thebottom of the chamber and a second pipe which terminates proximate tothe top of the chamber, wherein as oil is produced it is pumped into thechamber through said first pipe, displacing gas already in the chamberout through the second pipe. Preferably, then, said second plurality ofchambers are arranged in a cascade arrangement with each second pipeextending from proximate to the top of one chamber to the bottom of thenext chamber in the cascade so as to form the first pipe for said nextchamber. This arrangement has the advantage that the chambers of thecascade are filled or emptied in sequence rather than simultaneously,i.e. once the first chamber in the cascade is filled, it then overflowsinto the second. As a result, at any time only one of said secondplurality of chamber contains a mixture of oil and gas—the rest beingeither filled with gas or with oil, thereby reducing the free liquidsurfaces within the arrangement.

Preferably, the first plurality of chamber constitute substantially 80%of the chambers and the second plurality substantially 20% oil being fedor extracted from said first and second chambers simultaneously,preferably with substantially 80% of the flow being directed to saidfirst plurality of chambers and substantially 20% to said secondplurality. This has the advantage that, because the density of crude oilis substantially 80% that of sea water, a constant mass is maintained inthe tank as oil is loaded or unloaded from the arrangement whilst at thesame time the free surfaces of the fluids within the chambers isminimized, which is beneficial to the stability of the arrangement.

In an alternative embodiment, each chamber includes, in addition to saidfirst and second inlet/outlet pipes, a third inlet/outlet pipe whichterminates part way down the chamber, at a distance from the top of thechamber of substantially 20% of the height of the chamber. In thisembodiment, the first pipe provides an inlet/outlet for sea water, thesecond an inlet/outlet for gas and the third an inlet/outlet for oil,all chambers of the arrangement being equipped with an identical pipingarrangement and being fed simultaneously. A mixture of water and gas isthen used to ballast each chamber, each chamber being 80% filled withwater with gas thereabove when no oil is present, the volume of gaswithin each chamber being varied as oil is added/removed therefrom so asto ensure that the volume of water displaced by the oil as it is pumpedin is only 80% of the volume of oil. In this way, the overall mass ofthe arrangement is kept constant.

Some embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows a typical semi-submersible floating production system;

FIG. 2 shows in perspective a semi-submersible drilling rig with aconcrete tank attached in accordance with the present invention;

FIG. 3 shows a sectional elevation of the drilling rig of FIG. 2;

FIG. 4 is a schematic diagram of the layout of the connection betweenthe chambers of the concrete tank of FIGS. 2 and 3;

FIGS. 5 and 6 are further schematic diagrams showing how the chambersare connected;

FIG. 7 shows additional detail of the drilling rig of FIGS. 2 and 3; and

FIG. 8 shows an alternative gas/oil/water loading/offloadingarrangement.

Referring first to FIG. 2, the drilling rig 1 has a concrete tank 2attached below columns 30 and the pontoons 31.

As shown in FIG. 3, the concrete tank is segmented by fluid tight bulkheads in the form of internal concrete walls 3. At least one cell in thecentre of the tank is constructed to be open at top and bottom to createa hole through the centre of the tank whereby the rig can carry outdrilling or workover operations with the tank attached.

Also shown in FIGS. 3 and 5 for one chamber are the water inlet/outletpipe 4 and the oil inlet/outlet pipe 5. The water pipe terminates in adiffuser pipe 6 close to the bottom of the chamber which minimizesmixing of the oil and water as water is pumped in. The oil pipeterminates at the inner face of the upper surface to avoid thepossibility of build-up of a gas pocket. To maintain the mass of thetank and contents constant as oil is produced or offloaded only ⅘ of thechambers operate on the oil over water principle shown in FIG. 3.

FIG. 5 shows a piping arrangement which ensures longitudinal stabilityof the tank by ensuring the centre of mass remains suitably stationaryduring the loading/unloading process of the oil over water chambers. Theremaining ⅕ of the chambers utilize a gas over oil scheme employingpiping as shown in FIG. 6. Oil enters the first chamber A through oilpipe 9. As chamber A fills with oil, natural gas is vented via pipe 10to chamber B. When chamber A is full, oil will then travel through pipe10 to chamber B, displacing, in turn, the gas, via the link line intochamber C, and so on until all the chambers are full of oil and the gashas been vented from the final gas vent pipe 11. This process proceedsat the same time that ⅘ of the oil is being directed to the oil overwater chambers. Because the density of crude oil is approximately{fraction (4/5)} that of sea water this arrangement maintains a constantmass in the tank as oil is loaded or offloaded. These arrangementsminimize the free surfaces of liquids inside the chambers which isbeneficial to the stability of the vessel. FIG. 4 shows an examplelayout for the chambers with those operating a gas over oil schemearranged in a line along the centre of the tank and those operating awater over oil scheme being arranged symmetrically on either side so asto ensure lateral stability during the loading/unloading process.

An alternative arrangement to achieve this mass balance is shown in FIG.8. In this arrangement the loading system is identical for all chambers.Sea water inlet/outlet pipes are provided at the bottom of the chambers,oil inlet/outlet pipes are provided at ⅘ of the height of the chamberand natural gas inlet pipes are provided in the upper surface of thechambers. When no oil is on board, every chamber is filled ⅘ with seawater with natural gas above. As oil is produced it enters via oil pipe26 and sea water is displaced via water pipe 25. At the same time,natural gas is released via the gas inlet/outlet pipe 27 to ensure thatthe volume of water displaced by the oil is only ⅘ of the volume of oilentering.

The invention is further characterised by the fact that the mass of tankand contents is slightly greater than the buoyancy of the tank. Thismeans that to bring the combined semi-submersible/tank structure to thesame draft that the semi-submersible normally operates uncombined, someballast water must be removed from the semi-submersible causing afurther improvement in the metacentric height of the combined vessel.This arrangement creates a tensile force between the tank and rig.

A further characterization of the invention is that this tensile forcecan be changed to a compressive force which is beneficial to the fatiguelife of the vessel by eliminating any gap between the underside of thesemi-submersible pontoons and the upper surface of the tank.

By constructing the base of the tank of a material of greater densitythan the roof, the centre of gravity of the tank and contents isslightly below the centre of buoyancy of the tank. This increases themetacentric height of the vessel allowing an increased payload to beadded to the deck of the semi-submersible.

The volume of the tank is sufficiently large such that when the tank isempty, with the drilling rig attached above it, the upper surface of thetank is a considerable distance above the water line. Accordingly, eachchamber may be filled at least partially with gas, for example air, soas to reduce the total mass of the tank including its contents and hencereduce its draught so as to facilitate maintenance. This allows accessto the piping above the upper tank surface, to the manholes in the tankfor internal inspections, to the tank-to-semi-submersible connectionsand to all external parts of the semi-submersible. This allows theinspections required by classification societies to be carried outwithout the vessel needing to go to dry dock. The only parts of thevessel not inspectable in the dry are the underside and lower walls ofthe tank. Since the tank is constructed of concrete, periodic visualinspection by divers or remote underwater vehicle is consideredsufficient.

Concrete as a structural material behaves best when loaded incompression. Tensile forces must be resisted by reinforcing steelembedded in the concrete. The required quantity of reinforcing steel canbe minimized by maintaining the external pressure on the tank greaterthan the internal pressure. To achieve this the water outlet line fromeach oil-over-water chamber is connected to a breaktank 12 locatedinside a column of the semi-submersible as shown in FIG. 7. Thisbreaktank is located below the elevation of the operating water level(OWL). The breaktank is vented to atmosphere 13 and the water level inthe breaktank is maintained by level switches 14 and 15 acting on thesemi-submersible's seawater ballast pump 16 and control valve 17 to addor remove water as necessary. To ensure that the oil system cannotoverpressure the tank, the oil inlet/outlet is vented to atmosphere at asafe location 18.

Large centrifugal pumps 19 are required to offload the stored oil intoshuttle tankers. Such pumps require a net positive suction head (NPSH)on their suction side in order to function effectively. Normally this isachieved by locating the pumps at a lower elevation than the bottom ofthe storage tank. The arrangement of the present invention allows thepumps to be located above the top of the tanks in a caisson attached toone of the semi-submersible columns where they can easily be removed byone of the rig cranes for maintenance but still be provided with a netpositive suction head.

Also shown in FIG. 7 are an oil export meter 20, a ballast water cleanupdevice 21, an oil in water alarm 22 and a gas/oil/water separator 23.

The invention is also characterised by the fact that all necessaryvalving, pumps and instrumentation for the seawater system can belocated inside the columns of the semi-submersible where they are in adry, benign environment and can be easily accessed for maintenance.

The invention requires no pipes or fittings protruding from the bottomof the tank. This allows easy construction onshore, skidding of the tankinto the ocean, setting the tank on a suitable seabed for fitting to thesemi-submersible.

The invention is also characterised by the ability to construct theconnection between the tank and the semi-submersible rig in the dry eventhough this connection is underwater during normal operation. Onceconstructed the tank will be set on the seabed with a few metres ofwater above it. The semi-submersible at its minimum draft will befloated over the tank and deballasted down on to the top of the tank.The tank is then deballasted sufficient to lift the semi-submersibleclear of the water allowing the permanent connection to be constructedin the dry.

It will, of course, be understood that the proportion of 80% used aboveis connected to the relative density of sea water to crude oil and istherefore only an approximate value. Furthermore, in the event that thearrangement is utilized with liquids other than oil and/or water, therelative displacement quantities and/or the relative numbers of thefirst and second pluralities of chambers will then be in accordance withthe relative densities of the actual liquids used.

1. An oil storage assembly for a semi-submersible oil production vesselcomprising a deck structure, at least two underwater pontoons forproviding buoyancy to said deck structure, and a plurality of columnsconnecting said deck structure to said pontoons, comprising a concretetank is attached below said pontoons, said concrete tank beingsubdivided into a plurality of chambers for storing fluid.
 2. An oilstorage assembly according to claim 1, wherein the tank is subdividedinto a plurality of vertical chambers which form a matrix cross thetank, at least one of said chambers being open at the top and bottom soas to provide a through opening in the tank through which oil productionoperates can be carried out.
 3. An oil storage assembly according toclaim 2, wherein the tank is subdivided by means of a plurality of fluidtight bulkheads.
 4. An oil storage assembly according to any of claims 1to 3, wherein at least some of the chambers of the tank include a firstinlet/outlet conduit located proximate to the bottom of the chamber anda second inlet/outlet conduit located within substantially the upper 20%of the chamber.
 5. An oil storage assembly according to claim 4, whereina first plurality of said at least some chambers are arranged in acascade arrangement with the second inlet/outlet conduit of one of saidplurality of chambers connecting to the first inlet/outlet conduit ofthe next chamber in the cascade.
 6. An oil storage assembly according toclaim 5, wherein the first inlet/outlet conduit of the first chamber inthe cascade connects to an oil pipe and the second inlet/outlet conduitof the last chamber in the cascade connects to a gas pipe.
 7. An oilstorage assembly according to any of claim 6, wherein a second pluralityof said chambers have their first inlet/outlet conduit connected to afluid supply pipe and their second inlet/outlet conduit connected to anoil pipe, fluid being stored in said second plurality of chambers usingan oil over a suitable fluid scheme.
 8. An oil storage assemblyaccording to claim 7, wherein the fraction of said at least somechambers which form said second plurality is substantially equal to theratio of the density of oil to the density of the fluid over which it isstored in said second plurality of chambers, and the fraction of said atleast some chambers which forms said first plurality of chambers issubstantially equal to 1 minus the proportion which form said secondplurality.
 9. An oil storage assembly according to claim 8, wherein oilis stored over water in said second plurality of chambers, said firstplurality of chambers constituting 20% and said second plurality ofchambers 80% of said at least some chambers.
 10. An oil storage assemblyaccording to claim 4, wherein said second inlet/outlet conduit islocated substantially at the top of each said chamber.
 11. An oilstorage assembly according to claim 4, said second inlet/outlet conduitis located at a distance from the top of the chamber substantially equalto one fifth of the height of the chamber.
 12. An oil storage assemblyaccording to claim 11, wherein each of said at least some chambersincludes a third inlet/outlet conduit located substantially at the topof the chamber.
 13. An oil storage assembly according to claim 12,wherein said first inlet/outlet conduit connects to a water pipe, saidsecond inlet/outlet conduit to an oil pipe and said third inlet/outletconduit to a gas pipe.
 14. An oil storage assembly according to claim 4,wherein each said first inlet/outlet conduit has a diffuser assemblyprovided on its end so as to minimise the mixing of fluids within eachchamber.
 15. An oil storage assembly according to claim 1, wherein thetank is constructed of varying density materials such that the centre ofgravity of the tank and its contents is always lower than its centre ofbuoyancy, thereby causing an increase in the metacentric height of theassembly.
 16. An oil storage assembly according to any claim 1, whereinthe volume of the tank is such that when it is at least partially empty,its buoyancy is sufficient to maintain the production vessel at anelevation relative to the water at which the upper surface of the tankis above the waterline.
 17. An oil storage assembly according to claim1, further including an atmospherically vented break tank located insidethe vessel at an elevation below the operating water level by means ofwhich the internal pressure within the tank is maintained lower than theexternal pressure.
 18. An oil storage assembly according to claim 17,wherein one break tank is located inside a column of the vessel.
 19. Anoil storage assembly according to claim 17 or 18, wherein the fluidlevel in the break tank is controllable by a level controller whichactivates seawater ballast pumps to remove water and activates a controlvalve to allow addition of sea water from the ocean.
 20. An oil storageassembly according to claim 1, wherein a pump is provided to dischargeoil to a tanker, said pump being located inside one of said columnsabove the tank elevation.
 21. An oil storage assembly according to claim1, wherein the tank is constructed of reinforced and/or prestressedconcrete.
 22. A semi-submersible, floating production, storage andoffloading system for the development of offshore oil and gas fieldscomprising a drilling vessel, an oil storage assembly according to claim1, attached to the base of the drilling vessel, means for utilising thedrilling vessel's ballast pumps to add or remove water from the bottomof each chamber of the tank and means for directing produced oil into orout of the top of each chamber.
 23. A method of storing oil in anoffshore floating oil production facility comprising the steps ofattaching to the bottom of a pontoon structure a concrete tank, which issubdivided into a plurality of chambers, filling said chambers with atleast one fluid to adjust the buoyancy of the production facility, anddisplacing said fluid from said chambers by pumping produced oilthereinto in a controlled fashion such that the mass of the tank and itscontents is maintained substantially constant.
 24. A method according toclaim 23, comprising the further steps of adjusting the buoyancy of thefacility by filing a first plurality of said chambers with a first fluidand a second plurality of said chambers with a second fluid, pumping afirst proportion of produced oil into said first plurality of chambersso as to displace said first fluid therefrom, and pumping a secondproportion of said produced oil into said second plurality of chambersso as to displace said second fluid therefrom, said first and secondproportions being calculated based on the relative densities of theproduced oil and said first and second fluids so as to maintain asubstantially constant mass of fluid in said tank.
 25. A methodaccording to claim 24, comprising the further step of emptying oil fromsaid tank by pumping said first fluid back into said first plurality ofchambers and pumping said second fluid back into said second pluralityof chambers in relative amounts such that said first proportion of oilis displaced from said first plurality of chambers and said secondproportion of oil is displaced from said second plurality of chambers,wherein the mass of the tank and its contents is maintainedsubstantially constant during off-loading of the produced oil.
 26. Amethod according to claim 24 or claim 25, wherein said first fluid iswater and said second fluid is gas, said first plurality of chambersconstituting substantially 80% of the volume of the tank and receivingsubstantially 80% of the produced oil, and said second plurality ofchambers constituting substantially 20% of the volume of the tank andreceiving substantially 20% of the produced oil.
 27. A method accordingto claim 24, comprising the further steps of connecting said secondplurality of chambers in a cascade fashion such that fluid displacedfrom one chamber enters the next chamber in the cascade.
 28. A methodaccording to claim 23, comprising the further steps providing a waterinlet/outlet conduit substantially at the bottom of each chamber,providing a gas inlet/outlet conduit substantially at the top of eachchamber, and providing an oil inlet/outlet conduit substantially ⅕ ofthe height of the chamber from the top, filling each chamber of saidtank with a mixture of water and gas in order to adjust the buoyancy ofthe facility, pumping produced oil into each chamber by means of saidoil conduit to displace said water and gas so as to maintain eachchamber full of fluid and thereby minimise the free liquid surfacetherein, and controlling the relative proportions of water and gasdisplaced from said chamber by said oil in order to maintain the totalmass of the tank and its contents substantially constant.
 29. A methodaccording to claim 28, wherein oil is off-loaded from the tank bypumping water and gas back into each chamber in relative proportionssuch that the total mass of the tank and its contents remainssubstantially constant.
 30. A method according to claim 28 or claim 29,wherein said proportions in which said water and said gas are displacedfrom and/or pumped into each chamber are 80% water and 20% gas.
 31. Amethod according to claim 23, comprising the further steps of at leastpartially filling each chamber of said tank with gas so as to reduce thetotal mass of said tank including its contents and hence reducing itsdraught so as to facilitate maintenance.